Constraining Non-thermal Dark Matter by CMB

TL;DR

This paper investigates how early matter domination affects non-thermal dark matter abundance and uses CMB data to constrain such scenarios, especially for low annihilation rates and specific inflation models.

Contribution

It provides new bounds on the duration of early matter domination based on dark matter relic abundance and CMB spectral index constraints.

Findings

Lower bound on early matter domination duration for correct relic abundance.

Upper bound from spectral index constraints limits the epoch length.

Future CMB experiments can tightly constrain non-thermal dark matter models.

Abstract

A period of early matter domination can give rise to the correct dark matter abundance for a broad range of dark matter annihilation rate $\langle \sigma_{\rm ann} v \rangle_{\rm f}$. Here, we examine this scenario for situations where $\langle \sigma_{\rm ann} v \rangle_{\rm f}$ is below the nominal value for thermal dark matter $3 \times 10^{-26}$ cm$^3$ s$^{-1}$ as possibly indicated by some recent experiments. We show that obtaining the correct relic abundance sets a lower bound on the duration of early matter domination era in this case. On the other hand, provided that the post-inflationary universe has an equation of state characterized by $w \leq 1/3$, the requirement that the scalar spectral index $n_s$ be within the observationally allowed range limits the duration of this epoch from above. By combining these considerations, we show that the current and future cosmic microwave background experiments can tightly constrain the parameter space for this scenario. In particular, models of inflation with a tensor-to-scalar ratio below ${\cal O}(0.01)$ may disfavor non-thermal supersymmetric dark matter from a modulus-driven early matter domination epoch.